In some biological taxonomy schemes, protists (/ˈproʊtɨst/) are a large and diverse group of eukaryoticmicroorganisms, which belong to the kingdomProtista. There have been attempts to remove the kingdom from modern taxonomy but it is still very much in use.[1][2][3] The term Protoctista is also used for these organisms by various organisations and institutions.[4][5][6] Molecular information has been used to redefine this group in modern taxonomy as diverse and often distantly related phyla. The group of protists is now considered to mean diverse phyla that are not closely related through evolution and have different life cycles, trophic levels, modes of locomotion and cellular structures.[7][8] Besides their relatively simple levels of organization, the protists do not have much in common.[9] They are unicellular, or they are multicellular without specialized tissues; this simple cellular organization distinguishes the protists from other eukaryotes, such as fungi, animals and plants, although some fungi and animals are also unicellular[citation needed].

The term protista was first used by Ernst Haeckel in 1866. Protists were traditionally subdivided into several groups based on similarities to the "higher" kingdoms: the unicellular "animal-like" protozoa, the "plant-like" protophyta (mostly unicellular algae), and the "fungus-like" slime molds and water molds. These traditional subdivisions, largely based on superficial commonalities, have been replaced by classifications based on phylogenetics (evolutionary relatedness among organisms). However, the older terms are still used as informal names to describe the morphology and ecology of various protists.

The first groups used to classify microscopic organism were the Animalcules and the Infusoria.[10] In 1817, the German biologist Georg August Goldfuss introduced the word Protozoa to refer to organisms such as ciliates and corals.[11] This group was expanded in 1845 to include all animal-like unicellular organisms, such as foraminifera and amoebae. The formal taxonomic category Protoctista was first proposed in the early 1860s by John Hogg, who argued that the protists should include what he saw as primitive unicellular forms of both plants and animals. He defined the Protoctista as a "fourth kingdom of nature", in addition to the then-traditional kingdoms of plants, animals and minerals.[11] The kingdom of minerals was later removed from taxonomy by Ernst Haeckel, leaving plants, animals, and the protists as a “kingdom of primitive forms”.[12]

In 1938, Herbert Copeland resurrected Hogg's label, arguing that Haeckel's term protista included anucleated microbes such as bacteria, which the term "Protoctista" (literally meaning "first established beings") did not. In contrast, Copeland's term included nucleatedeukaryotes such as diatoms, green algae and fungi.[13] This classification was the basis for Whittaker's later definition of Fungi, Animalia, Plantae and Protista as the four kingdoms of life.[14] The kingdom Protista was later modified to separate prokaryotes into the separate kingdom of Monera, leaving the protists as a group of eukaryotic microorganisms.[15] These five kingdoms remained the accepted classification until the development of molecular phylogenetics in the late 20th century, when it became apparent that neither protists nor monera were single groups of related organisms (they were not monophyletic groups).[16]

Some protists, sometimes called ambiregnal protists, have been considered to be both protozoa and algae or fungi (e.g., slime molds and mixotrophic algae), and names for these have been published under either or both of the ICN and the ICZN.[17][18]

Phylogenetic and symbiogenetic tree of living organisms, showing the origins of eukaryotes

Although systematists today do not treat protists as a formal taxon, the term protist is currently used in two ways. The most popular contemporary definition is a phylogenetic one, that identifies a paraphyletic group: a protist is any eukaryote that is not an animal, (land) plant, or (true) fungus; this definition excludes many unicellular groups, like the Myxosporida (animals), the Microsporidia (fungi), many Chytridiomycetes (fungi), and yeasts (fungi). The other definition describes protists primarily by functional or biological criteria: protists are essentially those eukaryotes that are never multicellular,[19] that either exist as independent cells, or if they occur in colonies, do not show differentiation into tissues;[20] this definition excludes the brown algae, and many red and green algae. The term protozoa is used to refer to heterotrophic species of protists that do not form filaments. These terms are not used in current taxonomy, and are retained only as convenient ways to refer to these organisms.[citation needed]

Because the protists as a whole are paraphyletic, new systems often split up or abandon the kingdom, instead treating the protist groups as separate lines of eukaryotes. The recent scheme by Adl et al. (2005)[20] is an example that does not bother with formal ranks (phylum, class, etc.) and instead lists organisms in hierarchical lists. This is intended to make the classification more stable in the long term and easier to update. Some of the main groups of protists, which may be treated as phyla, are listed in the taxobox, upper right.[29] Many are thought to be monophyletic, though there is still uncertainty. For instance, the excavates are probably not monophyletic and the chromalveolates are probably only monophyletic if the haptophytes and cryptomonads are excluded.[30]

Nutrition can vary according to the type of protist. Many protists are flagellate, for example, and filter feeding can take place where the flagella find prey. Other protists can engulf bacteria and other food particles, by extending their cell membrane around them to form a food vacuole and digest them internally, in a process termed phagocytosis.

Some species, for example Plasmodium falciparum, have extremely complex life cycles that involve multiple forms of the organism, some of which reproduce sexually and others asexually.[31] However, it is unclear how frequently sexual reproduction causes genetic exchange between different strains of Plasmodium in nature and most populations of parasitic protists may be clonal lines that rarely exchange genes with other members of their species.[32]

Eukaryotes emerged in evolution more than 1.5 billion years ago.[33] The earliest eukaryotes were likely protists. Although sexual reproduction is widespread among extant eukaryotes, it seemed unlikely until recently, that sex could be a primordial and fundamental characteristic of eukaryotes. A principal reason for this view was that sex appeared to be lacking in certain pathogenic protists whose ancestors branched off early from the eukaryotic family tree. However, several of these protists are now known to be capable of, or to recently have had the capability for, meiosis and hence sexual reproduction. For example, the common intestinal parasite Giardia lamblia was once considered to be a descendant of a protist lineage that predated the emergence of meiosis and sex. However, G. lamblia was recently found to have a core set of genes that function in meiosis and that are widely present among sexual eukaryotes.[34] These results suggested that G. lamblia is capable of meiosis and thus sexual reproduction. Furthermore, direct evidence for meiotic recombination, indicative of sex, was also found in G. lamblia.[35]

The pathogenic parasitic protists of the genus Leishmania have been shown to be capable of a sexual cycle in the invertebrate vector, likened to the meiosis undertaken in the trypanosomes.[36]

Trichomonas vaginalis, a parasitic protist, is not known to undergo meiosis, but when Malik et al.[37] tested for 29 genes that function in meiosis, they found 27 to be present, including 8 of 9 genes specific to meiosis in model eukaryotes. These findings suggest that T. vaginalis may be capable of meiosis. Since 21 of the 29 meiotic genes were also present in G. lamblia, it appears that most of these meiotic genes were likely present in a common ancestor of T. vaginalis and G. lamblia. These two species are descendants of protist lineages that are highly divergent among eukaryotes, leading Malik et al.[37] to suggest that these meiotic genes were likely present in a common ancestor of all eukaryotes.

Based on a phylogenetic analysis, Dacks and Roger proposed that facultative sex was present in the common ancestor of all eukaryotes.[38]

This view was further supported by a study of amoebae by Lahr et al.[39] Amoeba have generally been regarded as asexual protists. However these authors describe evidence that most amoeboid lineages are anciently sexual, and that the majority of asexual groups likely arose recently and independently. It should be noted that early researchers (e.g., Calkins) have interpreted phenomena related to chromidia (chromatin granules free in the cytoplasm) in amoeboid organisms as sexual reproduction.[40]

Protists generally reproduce asexually under favorable environmental conditions, but tend to reproduce sexually under stressful conditions, such as starvation or heat shock.[41] Oxidative stress, which is associated with the production of reactive oxygen species leading to DNA damage, also appears to be an important factor in the induction of sex in protists.[41]

Some protists are significant pathogens of animals and others are pathogens of plants; as an example of the former, five species of the parasitic genus Plasmodium cause malaria in humans and many others cause similar diseases in other vertebrates; as an example of the latter, the oomycetePhytophthora infestans causes late blight in potatoes.[42] A more thorough understanding of protist biology may allow these diseases to be treated more efficiently.

Recent papers have proposed the use of viruses to treat infections caused by protozoa.[43][44]

Researchers from the Agricultural Research Service are taking advantage of protists as pathogens to control red imported fire ant (Solenopsis invicta) populations in Argentina. Spore-producing protists such as Kneallhazia solenopsae (generally recognized as belonging to the fungus kingdom now) can reduce red fire ant populations by 53–100%.[45] Researchers have also been able to infect phorid fly parasitoids of the ant with the protist without harming the flies. This turns the flies into a vector that can spreads the pathogenic protist between red fire ant colonies.[46]

More probable eukaryote fossils begin to appear at about 1.8 billion years ago, the acritarchs, spherical fossils of likely algal protists.[65] Another possible representant of early fossil eukaryotes are the Gabonionta.